The present disclosure generally relates to communication systems, and more particularly to estimating the frequency error on a received signal that is immune to FM clicks.
A problem that plagues almost all frequency modulated (FM) and frequency shift keyed (FSK) radio systems are the FM “clicks” and “pops” that occur at low signal levels, such as when the channel signal to noise ratio (SNR) gets to 10 dB or less. “Clicks” and “pops” are generally understood to be noise events that enhance the additive noise generated in the demodulation process. At low input signal-to-noise ratios, the resulting noise enhancements can become the dominant noise source.
Frequency demodulation is normally accomplished by comparing the phase of a signal at one instant in time to the phase of the signal at an adjacent instant. The frequency of a signal is the change in phase divided by the change in time. Modulation on the signal will push the phase back and forth. By detecting these changes, FM is demodulated. The FM signal can generally be pictured as a constant-amplitude vector that pivots up and down. Noise pushes the signal vector about, but the average location of the vector remains the same, so post-demodulation filtering can remove most of the effects of noise. However, when the signal gets weak, for example at around 10 dB SNR, there is a possibility of noise making it appear as though the vector took a 360 degree spin about the origin, which it did not. This 2-π (pi) rotation causes an impulse in the demodulated output waveform, which is generally in the form of a large pulse having a frequency content extending over a wide bandwidth including DC. Filtering the signal merely spreads the pulse out over multiple samples. At modulation indexes typical of narrowband data radios, the actual phase never moves much more than 45 degrees in a bit time. So while post-detection filtering averages out regular FM noise, click or pop-noise does not benefit from any amount of filtering. It is common for an FSK modem's performance to be surprisingly poor, even though the post detection SNR is quite good, because the pre-detection SNR is under 10 dB, resulting in clicks.
For example, in a high performance modem for narrow-band UHF channels, at low bit rates, frequency error relative to the symbol rate is significant. At 4800 bits per second on a 512 megahertz (MHz) radio having a 1 ppm net frequency error between transmit and receive, the phase will move approximately 38 degrees per bit due to the frequency error alone. This error must be accounted for. One approach is to estimate the frequency error with a conventional limiter-discriminator followed by a filter to average the waveform over a few dozen bit times. However, clicks will cause an error in the frequency estimate if we try to operate below a 10 dB SNR. Since certain wireless devices with optimized forward error correction are capable of performing with channel SNR below 0 dB, it would be advantageous to keep the frequency estimation technique from limiting the sensitivity.
Another approach to minimizing FM clicks involves a phase locked loop (PLL) to demodulate the signal. By tailoring the bandwidth of the loop, one can make the loop just wide enough to demodulate the signal, but not wide enough to follow the 2-π phase change produced by a click. This is usually referred to as “threshold extension” because it permits demodulation below the usual FM threshold, around 10 dB SNR. However, a phased locked loop demodulator adds complexity to radio receiver systems that is not common in commercial and consumer electronics.
Accordingly, it would be desirable to provide a system that addresses at least some of the problems identified.